Yellow earth is found in many places as compact masses, and less frequently as schistous deposits. It has a fine earthy fracture, and is mostly devoid of lustre, except for a faint shimmer on the surface of fracture; slightly greasy feel; and a tendency to crumble, in water, to a non-plastic powder. It contains silica, ferric oxide and water in varying proportions, and the yellow earths from different deposits always vary slightly in percentage composition. These differences are clearly shown in the following analyses of two varieties from the vicinity of Amberg (Bavaria):—
| I. | II. | |
| Silica | 33·23% | 35·10% |
| Alumina | 14·21 | 14·40 |
| Magnesia | 1·38 | — |
| Ferric oxide | 37·76 | 36·80 |
| Water | 13·24 | 13·60 |
When heated, the colour changes gradually to red, and the earth becomes extremely hard. There are several recognised commercial grades, the price of which varies mainly in accordance with the colour and fineness. The Amberg variety is specially esteemed, the Hungarian and Moravian kinds being less valuable.
The colour not being particularly good, this earth is never used for fine work, but is largely employed as a yellow wash for houses and as ordinary distemper. It may also be used as an oil paint.
Red Ochre is a less important, cheap variety of ochre, chiefly used in cheap paints and for low-priced wall-papers. It occurs in the deposits as clayey masses.
Terra di Siena
Terra di Siena is a very pure form of ferric hydroxide. When ground, the light to dark brown lumps furnish a pale to dark yellow powder, which can be transformed into a number of gradations by burning. In spite of its handsome colour, this pigment is deficient in covering power, in addition to which it darkens when mixed with varnish, and dries slowly.
(C) The Red Earths
Apart from the small quantities of native vermilion handsome enough for direct use as painters’ colours when reduced to powder, the red earths, with practically no exception, consist of ferruginous minerals, and it is only within a recent period that red painters’ colours have been prepared from certain chemical waste products from manufacturing processes. In all cases, however, compounds of iron and oxygen constitute the bulk of the red earths. In addition to ferric oxide, which is the chief material used for making the important red colours, are compounds of ferric oxide and water, i. e. ferric hydroxides. The ferric oxide pigments are among the most important in the entire series of earth colours, being on the one hand very cheap, and on the other so handsome in colour that ferric oxide can be used for the finest paintings.
Ferric oxide can also be shaded very extensively by a fairly simple treatment, so as to furnish a whole range of very handsome shades.